1. Field of the Invention
The present invention relates to an over-power protection apparatus, and more particular to an over-power protection apparatus used in a power supply for programming an over-current threshold.
2. Description of Related Art
Various power converters have been widely used to provide a regulated output voltage and current. For the sake of safety reasons, an over-power protection means must be provided to protect both the power converter itself and the power system. A limited power output is thus required for the power converter during the conditions for overloading, short-circuit and feedback open-loop.
Referring to FIG. 1 for the circuit diagram of a prior art power converter, the power converter comprises a transformer T1, a control unit 14, a driving unit 10, an oscillator 12, a current sense resistor RS, and a power switch Q1. The output terminal of the power converter can obtain a voltage feedback signal VFB. A current sense signal VCS is produced by a primary switching current IP of the transformer T1 through a current sense resistor RS. The control unit 14 receives the current sense signal VCS, a maximum power signal VLMT, and the voltage feedback signal VFB. The clear signal CLR outputted by the control unit 14 is generated when the current sense signal VCS is higher than the maximum power signal VLMT or the voltage feedback signal VFB. The driving unit 10 receives a pulse signal CLK outputted by the oscillator 12 and a clear signal CLR outputted by the control unit 14 for controlling the driving unit 10 to periodically disable the driving signal VPWM. Therefore, the output of the power converter is regulated, and the primary switching current IP of the transformer T1 and the maximum output power are limited further.
In practical application, the power converter is used in printers and scanners having a motor load that must provide a peak power for a short period. In other words, the peak power greater than the over-current threshold is permitted within a predetermined period to meet a larger output power and a starting torque for the motor load needed. The peak power is always equal to several times of the maximum output power, such that the power switch and the other power devices must increase the voltage and current stresses, and thus increasing the cost. Therefore, it is an important to provide an appropriate protection and to avoid an erroneous judgment for the power converter. Since the pin number in the integrated circuit is limited to satisfy the commercial specification, therefore the peak power is usually set to a constant value according to the load requirement, and there is no additional pin to adjust the over-current threshold. Such power converter is provided for a specific load, but it is not applicable for a common use in the industry application.